This invention relates to solar energy collectors and, more particularly, to improvements in the type of collector which is adapted to convert solar radiation directly to thermal energy in the absorber which, in turn, conducts heat directly to a fluid medium such as water.
In recent years, there have been numerous proposals and attempts to develop and improve solar energy collectors so that their efficiency and cost may approach a level which is economically feasible. While some progress has been made, the efficiency of the collectors in the state of the art still leaves much to be desired. In general, the efficiency of solar energy collectors has been hampered by difficulties encountered with suppressing the substantial portion of absorbed energy escaping by conduction through the transparent cover.
In general, the typical installation includes a plurality of collectors mounted on top of a roof or other portion of a building and connected to the building's heating system or other system adapted to use or store hot water. A collector typically includes a housing which is provided with thermal insulation about its bottom and sides. A cover, which is transparent to solar radiation, is provided on top of the device. A solar energy absorber is disposed within the housing, below the cover, so that it may be exposed to and absorb the solar radiation which passes through the cover. The typical absorber is in the form of a thermally conductive plate, such as an appropriate metal, which may be provided with an absorption-enhancing coating. Attached to or formed integrally with the plate are a plurality of spaced tubes which define comparatively large cross sectional flow channels for the fluid medium such as water. Typically, the tubes are disposed along the absorber plate in a relatively wide spacing, the space between the tubes being a number of times greater than the diameter of the tubes. Most of the impinging solar energy must be absorbed by the intermediate webs of the plate and then conducted through the connective webs to the tubes which are intended to heat the fluid flowing through the tubes.
Among the factors which lead to the hot spots is that the relatively wide webs are relatively thin and therefore present significant resistance to lateral heat transfer to the fluid tubes. In addition, the geometry of the prior devices do not provide optimum heat transfer area between the absorbing surface and the fluid which tends to restrict the rate of heat transfer to the water. Also, the typical prior devices utilize fluid flow channels which are of relatively large cross sectional area and carry relatively large masses of water which reduces the rate of heat transfer at least to those portions of the water contained within the innermost regions of the flow channels.
In general, it is believed the primary difficulty with prior devices had its origin in the designer's idea of the problem, resulting in misplaced emphasis. All prior art "high efficiency" collectors employ some costly modification of the airspace by added material structures, or a vacuum, or use special cover coatings. Such measures have the object of supressing the convection and radiation thermal paths from absorber surface to cover. The idea that better results might be obtained through geometry changes for a very low resistance conductive path to the fluid has not been in evidence.
It is among the objects of my invention to obtain through geometry along the lowest resistance path possible for absorbed energy to reach the circulating fluid, while at the same time holding the cover at the lowest possible temperature, that is near fluid temperature, for lowest cover conduction loss.